In a conventional ink-jet printer, sheets are directed through a print cycle which includes picking up a sheet from an input tray, feeding it through a printing zone for printing, and then expelling it through an output port. Once expelled, sheets fall to an output tray, consecutive sheets piling one on top of the other so as to form an output stack. Because ink-jet printers print using wet ink, and because sheets often are stacked immediately after printing, ink-jet printers have in the past experienced some difficulty with blotting and/or smearing of ink upon contact between consecutively printed sheets. This has been particularly apparent where ink drying time exceeds the time between printing of consecutive sheets. Although a variety of solutions have been proposed to deal with this problem, none have provided adequate ink drying time without some cost to the printer's efficiency, versatility or size.
Some manufactures have, for example, attempted to eliminate ink smearing and blotting problems by decreasing ink drying time. Manufacturers thus have employed quick-drying ink, or specially-coated paper, often resulting in poorer quality print. Manufacturers also have sought to provide some sort of drying lamp or heater adjacent the printed media, thus adding to the complexity of the printer, and consequently adding to the printer's price.
Other manufacturers have attempted to delay the deposit of printed sheets in the output tray so as to provide the previously-printed sheet with adequate drying time. The most basic of such solutions have involved simply slowing printer throughput by creating an artificial time delay between printing of consecutive sheets. Although this solution does increase the time available for ink to dry, it has proven unacceptable in view of the ever-increasing desire to improve printer efficiency and speed.
Another solution proposed by printer manufacturers has been to employ a passive sheet media drop scheme whereby a sheet emerging from the printer's output port is guided along rails which temporarily support the sheet above the output tray. Upon completion of printing, the sheet simply drops under its own weight into the output tray, the previously-printed sheet having had ample opportunity to dry during printing of the present sheet. However, such passive drop schemes are not always reliable due to a phenomenon known as cockling, an effect which may result in undulation of a printed sheet due to environmental extremes or large amounts of ink on the sheet. As a result of such cockling, sheets do not always drop into the output tray after printing, but instead are pushed forward and out of the printer by the following sheet.
Yet another solution has involved the use of an active sheet media drop mechanism wherein a printed sheet is guided along a pair of movable rails which temporarily support the sheet above the printer's output tray while the previously printed sheet drys. Once printing is completed, the rails retract, often pivotally, allowing the sheet to fall to an output tray below. One such active drop mechanism is described in U.S. Pat. No. 4,794,859 to Huseby et al., which is entitled "Active Paper Drop for Printers", and which is commonly owned herewith. The disclosure of that patent is incorporated herein by this reference.
Although generally effective, active drop mechanisms generally have presented problems in known sheet media handling systems due to difficulties associated with the support of sheets of different size. Because of the different sheet sizes, rails have in the past been made relatively wide such that rails of a fixed dimension could support variously-sized sheets. This wide rail design, in turn, has increased printer chassis size, and has required an unnecessarily large amount of raw material to manufacture the chassis and rails. Use of wide rails also has led to increased sheet sail, due in part to the wider sweep of such rails when an expelled sheet is released. The present invention provides an improved sheet media handling system which affords active release of variously-sized sheets without unduly increasing the system's size, complexity or price.